In the WDM transmission system, cross talks must be prevented by keeping the spacing of wavelengths between adjacent optical signals constant, and technologies on this subject are disclosed on Japanese Patent Kokai Nos. 2-228830 and 4-335724. In these technologies, the absolute wavelengths of the optical signals are always kept constant by separately controlling the respective wavelengths of the optical signals.
In the conventionally proposed WDM optical transmission system, in which the plural optical signals with the different wavelengths are transmitted through the common optical fiber transmission line using the conventional wavelength stability circuits, the absolute wavelengths of the optical signals to be transmitted are controlled, but the spacing of the wavelengths between the adjacent optical signals are not controlled at all.
In situations where the spacing of the wavelengths between the adjacent optical signals becomes very small in order to increase the multiplexed density by increasing the number of the optical signals, it is necessary to decrease the cross talks between the optical signals by keeping the spacing of the wavelengths between the adjacent optical signals larger than a critical value.
However, since the absolute wavelengths of the optical signals are controlled in the aforementioned conventional technologies, the spacing of the wavelengths between adjacent optical signals can not become constant, so that the generation of the cross talks can not be suppressed.